Square root biased sampling isn’t a technique that’s widely used, and it’s doubtful that you’ll be tested on it in any elementary statistics or AP statistics class. That said it is an interesting technique that attempts to address the problem of profiling at airport screenings.
I get selected for “extra screening” every time I travel by plane. I’m guessing it’s because I have dreadlocks, but I really have no idea. All I know is something about me is causing security to pull me aside every time. As well as it not being fair, it’s also taking up resources that could be better spent looking at other people who might actually be up to terrorist activities!
Statisticians strive to choose completely random people for surveys and experiments. This completely random sampling doesn’t happen at airport screenings (presumably because people who “look” a certain way are more likely to be terrorists), and this is a problem William H. Press attempts to address with square root biased sampling. He states:
…resources are wasted on the repeated screening of higher probability, but innocent, individuals.
In other words, profiling by ethnicity, having the same name as someone on a watch list, or in my case — having dreadlocks — isn’t a mathematically sound way to catch a terrorist.
Square root biased sampling adds simple random sampling to profiling. Simple random sampling is where individuals are chosen completely randomly and by chance from a population. The addition of SRS increases the chance a guilty person will be found and lets innocent travelers more likely to breeze through security. The system works by assigning the same profiling, but instead of a profiled passenger being selected for screening every time, they may be pulled aside less frequently. For example, if a person is 10 times more likely to be a terrorist, the current system would pull them aside ten times more often than a non-profiled passenger (which basically amounts to every time that profiled person travels they will be pulled aside). The addition of SRS means that the passenger will only be pulled aside three times as often.
You can find the full article by William Press here.